The present invention related to improvements in strain-gage fluid-pressure transducers, and, in one particular aspect, to novel and improved diaphragm-type pressure cells of low-cost uncomplicated construction which lend themselves to highly-automated fabrication in a miniature standardized configuration to sense and precisely signal fluid pressures stably and over wide ranges.
Strain-gage pressure transducers have long been known in a variety of forms wherein pressure-related deformations of diaphragms, thin-walled tubing, and the like, have been converted into related changes in resistance of gages disposed in arms of an electrical bridge network. An early development, as disclosed in U.S. Pat. No. 2,327,935--Simmons, utilized a strain-sensitive filament about a hollow thin-walled stud which expanded as the result of explosive impingements of fluid pressure forces upon a piston carried at the end of the stud. Simultaneous responses to both compression and tension strains on one side of a diaphragm were also realized in sophisticated pioneer work according to U.S. Pat. No. 2,400,467--Ruge, where a thick-rimmed gaged diaphragm capped a pipe exposed to pressures being sensed. A thin-walled internally-pressurized and externally-gaged tube was shown in the pressure gauge of U.S. Pat. No. 2,477,026--Wenk, Jr., et al, and a diaphragm bonded to the end of a pressure-probe tube and gaged on its interior was disclosed in U.S. Pat. No. 2,629,801--Warshaw and U.S. Pat. No. 3,305,818--Brueggeman et al. Electrical connections with the gages centered on the flexible portions of a diaphragm have also been facilitated, through use of extended flexible tab portions of a gage carrier, in U.S. Pat. No. 3,899,766--Mermelstein.
Underlying the present teachings is the recognition that it is the difficulties of producing pressure transducers economically, and in quantity, with reliable and stable high-precision characteristics, rather than any basic fault with the known general designs of such devices, which is a major obstacle to their more successful exploitation in a great variety of applications where they could serve to much advantage. In the case of the popular diaphragm-type transducers, for example, it can be quite troublesome to fabricate and install the diaphragms, if they are not integral parts of a transducer structure, and even when they are, it can likewise be awkward and onerous to locate and bond and calibrate minute gage networks on small surfaces which are not conveniently accessible. If the devices are to be of high technical quality, and yet inexpensive, it should also be possible to standardize their outputs readily and to compensate their gaging circuitry, without involving time-consuming, complex or painstaking skilled manufacturing operations. Moreover, it then becomes important to make electrical connections with and to protectively house and seal the sensitive gaged diaphragms without adversely affecting reliability of the devices and without detracting from their overall simplicity and ease of manufacture. In accordance with this invention, such advantageous features and aspects of improvement are promoted by way of a main transducer sub-assembly in which the diaphragm forms the integral bottom for a blind hole recessed into a flanged body from a pressure fitting, the outer side of the diaphragm and its coplanar annular rim forming an obstructed flat free end of the body. Diaphragm thickness may thus be mechanically adjusted readily from that end, to adapt the unit for optimum responses to selected ranges of pressures, and both the diaphragm and its coplanar rim are fully covered by a very thin insulating layer atop which foil gage elements are secured in position over the diaphragm and certain foil tabs and abradable compensation strips are secured in position over the stiff annular rim. Outwardly away from the flat diaphragm, and extending not beyond its flat end, is a peripheral cover-mounting shoulder onto which a second cooperating sub-assembly may be fitted. The latter includes a hollow connector member which may be cupped over the flat free end of the first sub-assembly and provides anchoring for flexible electrical connections contacting the rim-mounted tab-like terminals for the gages. In a version wherein the transducer reference pressure is to be that of a near vacuum, the connector member is rigid and tightly sealed after evacuation; in another expression, the connector is of rubber and snap-locks over the transducer body.